At present, the known powder conveying devices are operated based on Venturi Principle, which carry powder by airflow. The known powder conveying devices are simple in structure, but have some deficiency. Firstly, the concentration of the powder delivered in the air flow is very low. Since the powder is conveyed in the airflow, the speed of carrying airflow has to be faster than the minimum suspension speed of the powder. Secondly, the amount of the conveyed powder will fluctuate over time. Furthermore, it is difficult to adjust the amount of powder delivery accurately. This deficiency is more pronounced when using a Venturi powder pump to convey powder coating after a period of time, both the film evenness of the coating and the visual quality of the surface finish will be affected.
Therefore, many alternative means of conveying powder other than using the Venturi Principle have been proposed in recent time. In recent years high dense phase conveying technology has been developed.
European Patent Application EP 1 106 547 A1 disclosed a device wherein powder is pneumatically transferred to a metering chamber that is connected to a powder suction conduit. The metering chamber further connects to a pressure conduit. The powder is conveyed from the metering chamber to a powder discharge conduit via the pressure conduit.
To make negative pressure in the powder suction conduit, the device requires negative pressure generated by external devices such as a vacuum pump outside the metering chamber. Control means has to be placed between the pump and the metering chamber. The control means is used to turn off or turn on the air conduit. The powder suction conduit and the metering chamber are separated by breathable membrane so that the powder flowed into the metering chamber cannot access the powder suction conduit. The deficiency of this proposal is that the breathable membrane will be clogged or blocked due to the composition and property of the powder, causing negative effect to the device's smooth operation.
European Patent Application EP0124933 disclosed another membrane pump for conveying powder, and described a pump having a powder sucking plunger that is movable up and down in a transfer chamber. When moving upwards, the powder sucking plunger generates negative pressure in the feed conduit and sucks the powder out of the storage container. The powder is then compressed in the transfer chamber with the help of downward movement of the powder sucking plunger. When the powder sucking plunger reaches the low stop point, the powder discharge conduit is open and the compressed powder is conveyed to an applying device. To generate negative pressure, the powder sucking plunger has to be sealed by a seal ring, causing moving parts vulnerable to wear and contamination. This kind of pump generates an uneven powder/volume flow. Furthermore, since the powder is firstly compressed prior to the conveying, the powder is probably compacted. For example, low temperature curing coating powder can easily block the transfer chamber.
That is why this kind of pump is never used for conveying coating powder.
According to U.S. Pat. No. 3,391,963, movement back and forth of the powder sucking plunger can prevent the membrane in a conveying device from being blocked. The powder in the membrane is shaken off because of the membrane's fluctuation. This device operates when compressed air is not fed to the transfer chamber. Since semipermeable membrane is required, the cost of the device is high, and the semipermeable membrane is vulnerable to wear. Furthermore, most of the powder is conveyed by the membrane's mechanical movement. Therefore, a part of powder still retains on the surface of the membrane and probably causes blockage over time.
Accordingly, a Chinese patent application, Publication No. CN 101992950A, entitled A Powder Conveying Method And Powder Conveying System For Smoothly Conveying Powder resolves the above mentioned problems. However, this powder conveying structure still has the following deficiency:
1) The powder inlet and powder outlet of the transfer chamber is configured at the bottom of the transfer chamber. Therefore, the subsequent inlet powder is blocked by the previous inlet powder. It is not helpful to suck more powder into the transfer chamber. That is, the powder inlet causing the problem of hindering powder inflow.
2) Powder of poor flowing property tends to retain in the bottom part of the transfer chamber. The powder is difficult to be normally conveyed and can easily block the powder channel.
3) When the powder pump conveys powder, the air flow is intermittently or continuously controlled, and affected by the intermitting operation of the powder sucking plunger. Therefore, the powder conveying is not consistent and not even.